The ability of a hybrid electric vehicle's energy storage system to accept large quantities of energy within short periods of time for storage and to quickly deliver stored energy in large quantities can improve the performance and fuel economy of such vehicles. Present day energy storage systems comprise special energy storage devices, such as batteries having sophisticated chemistry and/or devices sometimes referred to as “supercapacitors”. These devices can be generically described and constructed as high-voltage energy storage devices in contrast to the low-voltage battery or batteries present in the vehicle's low-voltage electrical system.
When the nature of vehicle operation involves frequent accelerations and decelerations, the energy storage system is subject to frequent cycling. The efficiency of energy recovery during vehicle braking depends on how much of the vehicle's kinetic energy can be accepted and stored by the energy storage system as the vehicle is decelerating. The ability of the energy storage system to contribute to vehicle propulsion torque depends on how much and how fast energy can be delivered from the energy storage system to the vehicle's drive train.
Sophisticated high-voltage batteries, using Lithium or Nickel Metal Hydride chemistries for example, are often used in hybrid electric vehicles instead of lead-acid storage batteries even though they are typically more expensive because they accept higher charge rates, enabling more efficient energy recovery and faster charging, and they have higher charge and discharge cycle endurance.
When an energy storage system also has a “plug-in” capability that enables it to be charged from an off-board electrical power grid providing AC power in an electric utility power format, such as 115-120-volt, 60-hertz, AC electricity from an electric utility company, an even more sophisticated and/or larger energy storage system may be used.
Certain vehicles may have a “plug-in” capability and rely extensively on energy obtained from an off-board power grid. The energy storage system of such a plug-in hybrid electric vehicle (PHEV) or a plug-in electric vehicle may be plugged into the electric utility grid during nighttime, when there is typically an excess of AC electricity available on the grid and the price per kilowatt-hour is typically at its lowest.
The energy storage capacity and the specific type and number of components of the energy storage system for any particular PHEV, or electric vehicle, may be chosen on the basis of how the vehicle is expected to be driven before the energy storage system is again re-charged.